Allow me to shine light on the situation.
While away from CR for a year I held 2 positions consecutively at Nationwide Arena. 1st as event security, gaurding the stages, backstage, what have you (advantages of being massive, and I don't neccessarily mean muscle mass). The 2nd was as an engineer. A good friend of mine is head engineer at the Arena and has allowed me to be in on many "secrets", a.k.a. the shit the public doesn't get to know. I was working the night she came to town. Every artist that comes to the arena from Brittany to you name it has requirements for their dressing rooms. In their contracts with the Arena, if the room is not stocked according to their specifications, they are allowed to refuse their performance. Such as security must allow them to take any girls they want backstage, and we did. I would be gaurding the door with a few others and band members from numerous groups would come out and start asking girls if they are over 18 and would take them backstage. Now that you have a somewhat general knowledge of how things are run at the arena let me show you Brittany's list of dressing room requirements while performing at the Arena in 2003. Keep in mind she had few (less than 15 that I saw, but thats a general figure) other people in the room with her throughout the night.
1) 2 5th's of Jack Daniel's
2) 2 CARTONS of Marlboro's
I shit you not and swear on my whole family's graves.
Although many rock artists do it, smoking does severely affect a persons voice, especially over and extended amount of time, BUT it is still possible to smoke and sound good in some cases. Does Brittany sound as good in concert as she does in her vid's and album's? Absolutely. Why is that? Easy. She doesn't always sing live. It's an act. Lip singing.